An important aspect of a cryogenic air separation system employing a distillation column is the condensation and vaporization system, and more particularly, the condensation of the higher pressure column vapor against reboiling of the lower pressure column bottom liquid to provide reflux for the columns and to provide an adequate up-flow of vapor through the structured packing in the lower pressure column. The reboiling of liquid oxygen is performed by heat exchange with nitrogen vapor from the top of the higher pressure column. During the heat exchange process, the nitrogen vapor is condensed, and at least some of the condensate is returned to the higher pressure column to act as a source of reflux for the higher pressure column. In some condenser-reboiler configurations, the heat exchange between the boiling liquid oxygen and the condensing nitrogen is carried out in a shell and tube heat exchanger with the liquid oxygen typically flowing within the tubes of the heat exchanger while the higher pressure column top vapor is processed on the shell side of the heat exchanger. Such shell and tube heat exchangers offer the advantage of improved operating characteristics from a safety perspective. Compactness of the shell and tube heat exchanger is achieved by having enhanced boiling and condensing surfaces, as generally described in U.S. Pat. Nos. 7,421,856; 6,393,866; and 5,699,671 and United States published patent application No. 2007/0028649.
There are two main types of heat exchangers used in the condensing-reboiling process including a thermosyphon type heat exchanger and a downflow heat type exchanger. In a thermosyphon type heat exchanger, the liquid oxygen liquid enters the tubes at the bottom and is vaporized as it passes up the tubes. In a downflow heat exchanger, the liquid oxygen liquid is vaporized as it flows downwardly within the tubes. While both of these configurations ensure safe operation of the oxygen vaporization process, both of these configurations also have certain disadvantages.
Other problems that diminish the thermal performance of the condenser-reboiler and, in turn, adversely affect the energy efficiency and operating costs of the cryogenic air separation unit are the accumulation of non-condensables in the main condenser-reboiler. The non-condensables, such as neon and helium, are present in very small quantities in air, but the accumulation of the non-condensables within a main condenser-reboiler results in a higher resistance to targeted heat transfer requiring a higher bulk temperature difference between the condensing nitrogen and boiling oxygen. As indicated above, the higher bulk temperature difference between the condensing nitrogen and boiling oxygen translates to a higher pressure requirement for the incoming nitrogen vapor which ultimately results in higher compression power and associated costs for the air separation unit. Unless the non-condensables are removed from the main condenser-reboiler cold heat exchange surfaces, the top temperature difference between the condensing nitrogen and boiling oxygen could be higher.
In addition, since the non-condensables tend to aggregate or accumulate on the heat transfer surfaces of the main condenser-reboiler where the bulk vapor velocities are lower, the high concentration zones of non-condensables in many current designs are dispersed throughout the main condenser-reboiler such that it becomes difficult to collect and remove them, which for some of the non-condensables such as neon which has significant commercial value, it cannot be recovered in a cost effective manner.
Accordingly, there is a need for an improved condensation and vaporization system which can be effectively employed to condense nitrogen vapor and vaporize liquid oxygen in a cryogenic air separation unit that does not suffer from the above-identified disadvantages.